Zooplankton dynamics in Lake Mendota: short-term versus long-term changes

SUMMARY.

• 1 A study of zooplankton dynamics was carried out in Lake Mendota, Wisconsin, for the years 1976–80. The data were compared with those of E. A. Birge, obtained in the years 1894–96.
• 2 The zooplankton annual cycles and sizes of standing crop were similar in both studies. There was more year-to-year variation within each study than there was between the two studies separated by over 80 years.
• 3 The data are discussed in relation to the prediction of long-term change in lakes. The value of a historical data set for long-term ecological studies is emphasized.

     

Nitrogen and phosphorus dynamics and retention in ecotones of Lake Titicaca,Bolivia/Peru

We are studying present conditions and consequences of material movement from land to water in the Lake Titicaca basin, and how fluxes are affected by human activities. The principal objective of this research is to describe and explain the variability in the Andean Altiplano of (a) water, nutrient and sediment fluxes from land and (b) composition, nutrient limitation and other important features of nearshore lake communities, and compare the effects of different agricultural practices (especially traditional and modern) on these factors. We are focusing on a comparison of the impacts of two forms of agriculture in this region: ancient raised fields currently under rehabilitation, and flat pastures and fields, which are more common. Results of the first year of study indicate there is substantial variability in nitrogen and phosphorus dynamics in relation to ecotone complexity (simple vs. intermediate vs. complex). Raised field sites have the beneficial effect of reducing high available nutrient concentrations (nitrate and soluble reactive phosphorus) and sediment load (measured as turbidity) as the water passes through them enroute to the lake. Aquatic vegetation (algae and macrophytes) reflect well ambient total nitrogen and phosphorus concentrations. Experimental nutrient limitation bioassays indicate that nitrogen is the most important limiting nutrient, though there is important spatial variability within the landscape, and phosphorus as well as nitrogen can be limiting.     

Neuregulins: functions,forms, and signaling strategies

The neuregulins (NRGs) are cell-cell signaling proteins that are ligands for receptor tyrosine kinases of the ErbB family. The neuregulin family of genes has four members: NRG1, NRG2, NRG3, and NRG4. Relatively little is known about the biological functions of the NRG2, 3, and 4 proteins, and they are considered in this review only briefly. The NRG1 proteins play essential roles in the nervous system, heart, and breast. There is also evidence for involvement of NRG signaling in the development and function of several other organ systems, and in human disease, including the pathogenesis of schizophrenia and breast cancer. There are many NRG1 isoforms, raising the question "Why so many neuregulins?" Study of mice with targeted mutations ("knockout mice") has demonstrated that isoforms differing in their N-terminal region or in their epidermal growth factor (EGF)-like domain differ in their in vivo functions. These differences in function might arise because of differences in expression pattern or might reflect differences in intrinsic biological characteristics. While differences in expression pattern certainly contribute to the observed differences in in vivo functions, there are also marked differences in intrinsic characteristics that may tailor isoforms for specific signaling requirements, a theme that will be emphasized in this review.     

Daphnia lumholtzi and Daphnia ambigua: population comparisons of an exotica and a native cladoceran in Lake Okeechobee, Florida

The population dynamics of an exotic cladoceran (Daphnialumholtzi Sars) and a native cladoceran (Daphniaambigua) were studied over a 12 month period in subtropical LakeOkeechobee, Florida (USA), to quantify the extent of invasion of the exoticspecies and compare ecological niches. Daphnialumholtzi accounted for up to 70% of theDaphnia assemblage during the summer months(June-August), while D.ambigua accounted for up to 97%of the Daphnia assemblage from fall to spring(October-April). The densities of the two species were inversely corelated.The exotic species was most concentrated in the shallower, warmer, northand south ends of the lake during the summer. It also was present, but atmuch lower densities, in the central lake region during the fall. Thenative species displayed a ubiquitous distribution throughout the lakeduring spring and winter, but was concentrated in the deeper, cooler,central region during the summer. Relationships of the two species withenvironmental conditions indicate that water column temperature mightaffect the seasonal and spatial distribution of the twoDaphnia species. The results also indicate thatD.lumholtzi may be filling a 'vacant' seasonal orspatial niche when conditions are unfavorable forD.ambigua.     

Revisiting the total maximum daily load total phosphorus goal in Lake Okeechobee

In 2000, a total maximum daily load (TMDL) total phosphorus (TP) goal of 40 µg/L was established for Lake Okeechobee, Florida. The goal was selected to reduce the “imbalance to flora and fauna” caused by excessive phosphorus loading in the lake. We recalculate the TMDL TP goal using a water quality data series of TP and chlorophyll-a concentrations from 1986 to 2018 for the original 30 in-lake stations plus an additional 29 stations. Using the cross-tabulation approach used to generate the original TMDL TP goal, we determined a new goal of 42 µg/L, nominally different from the original. We also reevaluate the goal’s ability to maintain an implicit goal of 11.5% bloom frequency. We conclude substantial changes in the frequency and scope of the water quality sampling scheme prevent a determination on the effectiveness of the TMDL TP goal. The implementation, however, appears to have failed as the median TP concentration has increased by 93 µg/L to 133 µg/L and bloom frequency, after correcting for the declining sampling frequency, has increased from 1986 to 2018. An increased sampling frequency of TP and chlorophyll-a sampling is needed, or else tracking responses of chlorophyll-a to future TP reductions will be virtually impossible.

     

Water quality effectiveness of ditch fencing and culvert crossing in the Lake Okeechobee basin, southern Florida, USA

Ditch fencing and culvert cattle crossing Best Management Practice (BMP) was evaluated in this study with regard to phosphorus (P) and nitrogen (N) load reductions and economic feasibility in the Lake Okeechobee (LO) basin. The BMP was implemented at a 170 m section of a drainage ditch within a ranch in the LO basin and flow and concentration (N and P) data at the upstream and downstream of the ditch were collected for one pre-BMP (June-October, 2005) and three post-BMP (June-October, 2006-2008) periods. During the pre-BMP period, downstream total P (TP) load was 20% (67.0 kg) higher than the upstream, indicating the cattle crossing to be a source of P. Downstream loads of TP in 2006 and 2008 (post-BMP periods) became 26% (14.7 kg) and 11% (85.9 kg) lower than the upstream loads, respectively indicating that the BMP reduced the P loads. The site was a sink for N for all periods except the 2007. Unusual dry conditions during 2007 resulted in the addition of P and N at the BMP site, probably due to the release of P and N from soil and plants. Average of three post-BMP period load showed a 10% reduction of TP loads at the downstream (251.8 kg) compared to the upstream (281.0 kg) location. To consider potential P contributions from the soil and plant, two scenarios, conservative and liberal, were considered to estimate P load reductions due to the BMP. For the conservative scenario, P contribution from soil and plant was considered, while for liberal it was not. Reductions in P loads for conservative and liberal scenarios were 0.35 and 0.44 kg/day, respectively. Phosphorus removal cost for the conservative scenario was $12.61/kg of P, which is considerably less than the cost of other P reduction strategies in the basin. Overall, results show that the BMP can reduce P concentration and loads from ranches without causing adverse impact on cattle production.     

Aquatic vegetation and largemouth bass population responses to water-level variations in Lake Okeechobee,Florida (USA)

A five-year study examined the responses of submerged aquatic vegetation (SAV), emergent vegetation, and largemouth bass (Micropterus salmoides) to variations in water level in a large lake in Florida, USA. SAV was assessed using a combined transect survey/spatial mapping approach, emergent vegetation was quantified with aerial photography and GIS, and bass were surveyed by electro-shocking. During the period leading up to this study (1995–1999), water levels were high in the lake, and the SAV was reduced in spatial extent and biomass, compared to its condition in the early 1990s. Spatial extent of emergent vegetation also was low, and largemouth bass surveys indicated low densities and failure to recruit young fish into the population. This was attributed to the lack of critical vegetative habitat. In spring 2000, the lake was lowered by discharging water from major outlets, and this was followed by a regional drought. Water levels dropped by 1m, and there was widespread development of Chara lawns in shoreline areas, with coincident increases in water clarity. There was some germination of vascular SAV, but Chara was the extreme dominant, such that structural complexity remained low. There was no substantive improvement in bass recruitment. During 2001, water levels declined further, and emergent plants germinated in exposed areas of the lake bottom. SAV was restricted to sites farther offshore, and continued to be dominated by Chara. There again was no bass response. In 2002, conditions changed when water levels increased to a moderate depth, flooding shoreline habitat to 0.5m. Vascular SAV increased in biomass and spatial extent, such that the community developed a high structural complexity. At the same time, emergent aquatic plants developed dense stands along the western shoreline. Largemouth bass had a strong recruitment of young fish for the first time in 5years. Recruitment continued to be successful in 2003, when spatial extent of SAV was somewhat reduced by higher water but total biomass and diversity remained high. These results demonstrate an important effect of inter-annual variation in water depth on the population dynamics of aquatic plants and fish in a subtropical lake.     

Periphyton nutrient limitation and other potential growth-controlling factors in Lake Okeechobee,U.S.A.

Periphyton nutrient limitation was assessed in Lake Okeechobee, a large, shallow, eutrophic lake in the southeastern U.S.A. Nutrient assays were performed to determine if the same nutrients that limit phytoplankton also limit periphyton growth in the lake. Nutrient diffusing clay substrates containing agar spiked with nitrogen, phosphorus, or both, along with nutrient-free controls, were incubated at four sites in the lake. Three sites were located in a pelagic–littoral interface (ecotone) and one site was located in the interior littoral region. Incubations lasted for 20–26 days, and were repeated on a quarterly basis between 1996 and 1997, to incorporate seasonal variability into the experimental design. The physical and chemical conditions at each site also were measured. Periphyton biomass (chlorophyll a and ash-free dry mass) was highest at the littoral and northern ecotone sites. At the littoral site, nitrogen limited biomass in four of five incubations, although the largest biomass differences between the treatments and controls (3 g cm^–2 as chl) were probably not ecologically significant. Periphyton biomass at the western and southern ecotone sites was low compared to the other two sites. Increases in water column depth and associated declines in light penetration strongly correlated with periphyton growth and suggested that they may have limited growth most often at all three ecotone sites. Nitrogen also was found to limit periphyton growth approximately 20% of the time at the ecotone sites and phosphorus was found to limit growth once at the west site.     

Comparative analysis of nutrients,chlorophyll and transparency in two large shallow lakes (Lake Taihu,P.R. China and Lake Okeechobee,USA)

This article compares limnological attributes of two of the world’s largest shallow lakes—Lake Okeechobee in Florida, USA and Lake Taihu in P.R. China. Both the systems support an array of ecological and societal values including fish and wildlife habitat, public water supply, flood protection, and recreation. Both have extensive research programs, largely because of concern regarding the lakes’ frequent cyanobacterial blooms. By evaluating these systems together, we compare and contrast properties that can generally advance the understanding and management of large shallow lowland lakes. Because of shallow depth, long fetch, and unconsolidated mud sediments, water chemistry, and transparency in both the lakes are strongly influenced by resuspended sediments that affect light and nutrient conditions. In the central region of both the lakes, where depth is the greatest, evaluation of limiting factors by a trophic state index approach indicates that light most often limits phytoplankton biomass. In contrast, the more sheltered shoreline areas of both the lakes display evidence of nitrogen (N) limitation, which also has been confirmed in nutrient assays conducted in earlier studies. This N limitation most likely is a result of excessive levels of phosphorus (P) that have developed in the lakes due to high external loads over recent decades and the currently high internal P recycling. Comparisons of these lakes show that Lake Taihu has higher N than, similar total phosphorus (TP) and similar light conditions to that of Lake Okeechobee, but less chlorophyll a (CHL). The latter may be as a result of lower winter temperatures in Lake Taihu (around 5°C) compared to Lake Okeechobee (around 15°C), which could reduce phytoplankton growth and abundance through the other seasons of the year. In these systems, the important role of light, temperature, and nutrients in algal bloom dynamics must be considered, especially due to possible adverse and unintended effects that might occur with projects such as sediment removal, and in the long term, in regard to buffering lake responses to external load reduction. Handling editor: D. Hamilton     

Mitochondrial morphology and dynamics: actors, mechanisms and functions

Mitochondria are dynamic organelles that continuously move, fuse and divide. Their overall morphology, ranging from a filamentous network to a collection of isolated dots, is determined by fusion-fission equilibrium, which depends on the cellular and physiological context. The machineries of fusion and fission, that are conserved throughout evolution, include three large GTPases of the dynamin-superfamily: Dnm1/DRP1 - involved in fission?- as well as Fzo1/MFN and Mgm1/OPA1?- required for fusion. While the activities, mecanisms and regulations of mitochondrial fusion and fission machineries continue to be unravelled, the relevance of mitochondrial dynamics is witnessed by their impact on organelle functions, cell survival and cell differenciation, their requirement for embryonic development and their involvement in neurological diseases.     

The influence of large water level fluctuations and hurricanes on periphyton and associated nutrient storage in subtropical Lake Okeechobee,USA

The abundance, community structure and nutrient content of periphyton, and the host plant taxa Chara, Hydrilla, Potamogeton, Vallisneria and Scirpus were studied in Lake Okeechobee, USA. Water levels were generally high during the study period (August 2002–January 2006), but substantial fluctuations occurred. All host plant biomass was seasonally variable but only Vallisneria biomass was spatially variable. All submerged plant beds disappeared after the passage of two hurricanes in September 2004, and a third hurricane passed over the lake in October 2005. Periphyton assemblages were statistically separated most by substrate and then by season. Prior to the hurricanes, annual maxima of periphyton biovolumes and those of summer submerged plant coverage coincided. During all study years, the diatom taxa dominated periphyton total biovolumes. Periphyton biomass was generally highest during the summer or prior to the hurricanes (in the case of epiphytes) and was spatially variable in the case of both Scirpus and Vallisneria. Epiphytic nutrient contents within each host plant group seasonally varied except for nitrogen and carbon in the Vallisneria epiphytes. Epipelic nutrient contents were spatially variable and seasonally variable for carbon. Nutrient contents in epipelon were significantly higher than that in Scirpus epiphytes and were similar but lower among all epiphytic communities. The total annual areal potential epiphytic phosphorus storage extrapolated during this study (2.0 × 10⁻⁴ metric tons ha⁻¹ year⁻¹) was underestimated because storage estimates for epipelon, Chara and Hydrilla-associated epiphytes were omitted. The Chara and Hydrilla-associated epiphytic nutrient storage values were omitted because of limited data, whereas the epipelic data may have not been spatially representative. For periphyton biovolume, host substrate type, water level fluctuation and hurricane impacts on host substrates appear to be more important than seasonal variation in such factors as temperature and nutrients. Epiphytic nutrient storage appears to be influenced most by water level fluctuation and hurricane-related impacts, while the host substrate type appears to be a less important factor than it is for periphyton biovolume. Maximum periphyton biomass and high nutrient storage in shallow subtropical and tropical eutrophic lakes may only occur at consistently lower water levels and during infrequent periods of disturbance, which enhance host substrate colonizable area.     

Viewing lip forms: cortical dynamics

Viewing other persons' actions automatically activates brain areas belonging to the mirror-neuron system (MNS) assumed to link action execution and observation. We followed, by magnetoencephalographic cortical dynamics, subjects who observed still pictures of lip forms, on-line imitated them, or made similar forms in a self-paced manner. In all conditions and in both hemispheres, cortical activation progressed in 20-70 ms steps from the occipital cortex to the superior temporal region (where the strongest activation took place), the inferior parietal lobule, and the inferior frontal lobe (Broca's area), and finally, 50-140 ms later, to the primary motor cortex. The signals of Broca's area and motor cortex were significantly stronger during imitation than other conditions. These results demonstrate that still pictures, only implying motion, activate the human MNS in a well-defined temporal order.     

Seasonal and spatial variation in zooplankton community structure and their relation to possible controlling variables in Lake Okeechobee

1. We examined the behavioural response of stream macroinvertebrates to real and simulated predatory activity by a river bird, the Eurasian dipper, Cinclus cinclus L.
2. In the field, we assessed whether invertebrate drift changed in response to live dippers in enclosures; we found no effects on the drift of any of the five families for which individual analyses were possible, either because it was infrequent, or involved distances too short to be detected (< 0.5 m).
3. In a laboratory stream, we observed prey during encounters with a model dipper which simulated flight, swimming, bill contact with the prey, and stone turning. Invertebrate families varied in their response. Simuliids and hydropsychid caddis lacked effective escape behaviour, consistent with heavy losses through predation by dippers in the wild. Other families either drifted (Baetidae, Gammaridae) or moved away (Heptageniidae, Ephemerellidae, Leuctridae, Perlidae) from the model dipper, but responded only to bill contact or simulated stone turning. Such delayed responses would not protect individuals directly targeted by foraging dippers and partly explain the lack of detectable effects by dippers on drift in the field.
4. We suggest why invertebrates do not show more marked escape responses to this important predator.     

Proteoglycans: many forms and many functions.

Proteoglycans are produced by most eukaryotic cells and are versatile components of pericellular and extracellular matrices. They belong to many different protein families. Their functions vary from the physical effects of the proteoglycan aggrecan, which binds with link protein to hyaluronan to form multimolecular aggregates in cartilage; to the intercalated membrane protein CD44 that has a proteoglycan form and is a receptor and a cell-binding site for hyaluronan; to heparan sulfate proteoglycans of the syndecan and other families that provide matrix binding sites and cell-surface receptors for growth factors such as fibroblast growth factor (FGF). One feature that recurs in proteoglycan biology is that their structure is open to extensive modulation during cellular expression. Examples of protein changes are known, but a major source of structural variation is in the glycosaminoglycan chains. The number of chains and their length can vary, as well as their pattern of sulfation. This may result in the switching of different chain types with different properties, e.g., chondroitin sulfate and heparan sulfate, and it may also result in the selective expression of sulfated chain sequences that have specific functions. The control of glycosaminoglycan structure is not well understood, but it does appear to be used to change the properties of proteoglycans to suit different biological needs. Proteoglycan forms of proteins are thus important modifiers of the organization of the pericellular and extracellular matrices and modulators of the processes that occur there.     

Seasonal and spatial variation in zooplankton community structure and their relation to possible controlling variables in Lake Okeechobee

1. We examined the behavioural response of stream macroinvertebrates to real and simulated predatory activity by a river bird, the Eurasian dipper, Cinclus cinclus L.
2. In the field, we assessed whether invertebrate drift changed in response to live dippers in enclosures; we found no effects on the drift of any of the five families for which individual analyses were possible, either because it was infrequent, or involved distances too short to be detected (< 0.5 m).
3. In a laboratory stream, we observed prey during encounters with a model dipper which simulated flight, swimming, bill contact with the prey, and stone turning. Invertebrate families varied in their response. Simuliids and hydropsychid caddis lacked effective escape behaviour, consistent with heavy losses through predation by dippers in the wild. Other families either drifted (Baetidae, Gammaridae) or moved away (Heptageniidae, Ephemerellidae, Leuctridae, Perlidae) from the model dipper, but responded only to bill contact or simulated stone turning. Such delayed responses would not protect individuals directly targeted by foraging dippers and partly explain the lack of detectable effects by dippers on drift in the field.
4. We suggest why invertebrates do not show more marked escape responses to this important predator.     

Seasonal changes in sediment phosphorus forms in relation to sedimentation and benthic bacterial biomass in Lake Erken

Surficial sediment and sedimenting material were sampled during spring and summer 1991 in Lake Erken. Sediment was analyzed for redox potential, P concentrations and bacterial biomass. Sedimentation and chlorophyll a concentrations of sedimenting matter were determined. Additionally, different phosphorus forms in surficial sediment were quantified using sequential fractionation. The resulting dataset was used to study the effects of sedimentation events following phytoplankton blooms and benthic bacterial biomass on the size of the various phosphorus pools in the sediment.Sedimentation of spring diatoms caused a rapid increase in the NH₄Cl- and NaOH-extractable P (NH₄Cl–P and NaOH–rP) in the sediment. During sedimentation, NaOH–rP and NH₄Cl–P increased within 3 days from 422 ± 17 g g^–1 DW to 537 ± 8.0 g g^–1 DW and from 113 ± 13 g g^–1 DW to 186 ± 26 g g^–1 DW, respectively. The NaOH–nrP (non-reactive P) fraction made up about 17% of Tot-P in sediment samples, whereas NaOH–rP and HCl–P made up 25% each. All P forms showed considerable seasonal variation. Significant relationships were found between bacterial biomass and the NaOH–nrP and NH₄Cl–P fractions in the sediment, respectively. Also regressions of NaOH–nrP and NH₄Cl–P versus the chlorophyll a concentration of sedimenting matter were highly significant. These regressions lend support to the conjecture that NaOH–nrP is a conservative measure of bacterial poly-P.     

Plant TGNs: dynamics and physiological functions

In all eukaryotic cells, a membrane trafficking system connects the post-Golgi organelles, including the trans-Golgi network (TGN), endosomes, and vacuoles. This complex network plays critical roles in several higher-order functions in multicellular organisms. The TGN, one of the important organelles for protein transport in the post-Golgi network, functions as a sorting station, where cargo proteins are directed to the appropriate post-Golgi compartments. The TGN has been considered to be a compartment belonging to the Golgi apparatus, located on the trans side of the Golgi apparatus. However, in plant cells, recent studies have suggested that the TGN is an independent, dynamic organelle that possesses features different than those of TGNs in animal and yeast cells. In this review, we summarize recent progress regarding the dynamics and physiological functions of the plant TGN.     

Phytoplankton of Lake Peipsi-Pihkva: species composition, biomass and seasonal dynamics

With 33 years of phytoplankton quantitative studies carried out, a series of qualitative data with a length of over 80 years is at our disposal. About 500 algal species have been found in plankton by different researchers. In different seasons and years 35 main species (dominants and subdominants) form 68–96 % of biomass in L. Pihkva (southern, more eutrophic part) and 60–97 % in L. Peipsi (northern, less eutrophic part). L. Lämmijärv, connecting the two parts is similar to L. Pihkva in respect to phytoplankton and the trophic state. Diatoms and blue-green algae prevail in biomass, diatoms and green algae, in the species number. The oligo-mesotrophic Aulacoseira islandica (O. Müller) Sim. is characteristic of the cool period; A. granulata (Ehr.) Sim. and Stephanodiscus binderanus (Kütz.) Krieger prevail in summer and autumn, the latter being most abundant in the southern part. Gloeotrichia echinulata (J.S. Smith) P Richter and Aphanizomenon flos-aquae (L.) Ralfs dominate in summer causing water-bloom. Phytoplankton has mostly three maxima in seasonal dynamics in L. Peipsi and two in L. Pihkva. Its average biomass in spring in different years has fluctuated in the range 5.6–16 and 6–12.7 g m^–3, in summer 3.1–14.8 and 5.6–125 (10–20 in most cases); and in autumn 7–16.3 and 5.2–26 in the northern and southern parts, respectively.The dominant complex has not changed considerably since 1909; however, the distribution of dominant species in lake parts has become more even in the last decades. Periods of high biomass occurred in the first half of the 1960s and 1970s and in 1988–1994, of low biomass in 1981–1987. The first coincided, in general, with periods of low water level and high water temperature.     

Extreme weather events influence the phytoplankton community structure in a large lowland subtropical lake (Lake Okeechobee, Florida, USA)

We demonstrate a major ecological change in a large lake ecosystem in response to a series of extreme weather events. Phytoplankton community dynamics in subtropical Lake Okeechobee are described from 2000 through early 2008 with emphasis on inter-relationships among phytoplankton populations and associated environmental variables in this large, shallow eutrophic lake. The lake experienced the physical effects of three hurricanes in 2004–2005, which caused massive sediment resuspension, near total elimination of submerged aquatic vegetation, elevated biologically available nutrients and total suspended solids, and lower water transparency. Patterns of long-term co-dominance by nitrogen (N)-fixing cyanobacteria and meroplanktonic diatoms abruptly changed to dominance by only meroplanktonic diatoms. The planktonic cyanobacteria genera Anabaena and Planktolyngbya both decreased approximately an order of magnitude in the post-hurricane period despite large surpluses of bioavailable nutrients. Meroplanktonic diatoms (mostly Aulacoseira spp.) declined approximately 20%, perhaps because of superior competitive ability for light in a polymictic, turbid water column. Canonical Correspondence Analysis (CCA) suggested that reduction in planktonic cyanobacteria after compression of the photic zone and the persistence of meroplanktonic diatoms were related to light utilization traits for the key algal taxa and indicated that pre-existing light limitation and crustacean grazing pressure were intensified in the post-hurricane period.     

Seasonal dynamics of picophytoplankton in Lake Kinneret, Israel

1. Picophytoplankton (picocyanobacteria and picoeukaryotes) communities in Lake Kinneret were studied from 1988 to 1992. No prochlorophytes were observed in the lake. 2. Picocyanobacteria were a prominent and ubiquitous component of the phytoplankton, being present at all depths throughout the year, with concentrations ranging from 2 ± 10–8 ± 10⁵ cells ml^?1. Low cell numbers in winter and spring were followed at the end of the annual dinoflagellate bloom by maximal abundances in summer-autumn in the epilimnion. High cell numbers (> 10⁴ cells ml^?1) were sometimes also found in the anaerobic hypolimnion. Net growth rates for picocyanobacteria ranged from 0.29 to 0.60 divisions day^?1. 3. Picoeukaryotes were a very minor constituent of the picoplankton, mostly present in winter and spring, and sometimes at the end of autumn, with concentrations ranging from 44 to 5700 cells ml^?1. Higher cell numbers tended to occur in the near surface water layers. In August-September, picoeukaryotes were found only in the hypolimnion. In December, the occurrence of picoeukaryotes in the deep water layers probably resulted from advection with cold water currents from the Jordan river. Net growth rates for picoeukaryotes ranged from 0.26 to 0.43 divisions day^?1. 4. Overall, the contribution of picophytoplankton to the phytoplankton standing crop in Lake Kinneret was limited; picocyanobacteria and picoeukaryotes accounted for no more than 7.0 and 0.1% of total algal biomass (semiannual average), respectively. 5. Picophytoplankton cell numbers in pelagic waters were usually similar to those in shallower lake stations. 6. Picocyanobacteria appear to be an autochthonous population, whereas picoeukaryotes are probably brought annually by the Jordan River and do not maintain themselves in the lake.